Electrically conductive compositions and methods for producing same

ABSTRACT

An electrically conductive composition and a method for producing same, for example by injection molding. The composition includes a matrix comprising substantially a first thermoplastic component, a second thermoplastic component having a higher polarity than that of the first thermoplastic component, the second component encapsulating a plurality of fibers forming a network of encapsulated fibers within the matrix, and a carbon component preferentially attracted to the second component so as to make said network an electrically conductive network within the matrix.

This application claims the benefit of provisional application Serial No. 60/030,621, filed Nov. 7, 1996.

FIELD OF THE INVENTION

The present invention relates to electrically conductive composites based on a mixture of two thermoplastic compounds having different polarity with glass fibers and carbon black and or carbon fibers and to methods for producing same.

BACKGROUND OF THE INVENTION

Electrically conductive polymer based compositions are used in many industrial applications, such as for dissipating electrostatic charge from plastic parts and plastic boxes for shielding electronic components from electromagnetic interference (EMI). Examples for electrostatic discharge (ESD) applications are electronic packaging, clean room installations, storage trays, water carriers, chip carriers and construction components for explosion-proof environments.

Compounds tailored for dissipating static electricity having a typical surface resistivity of 10² to 10¹³ ohm/square and compounds specified for EMI shielding applications typically exhibit volume resistivity of 10⁻² to 10² ohm-cm.

There are known in the art polymer based compounds having appropriate resistivity for both static electricity dissipation and EMI shielding. One class of compounds known in the art is based on polypropylene (PP) or polyethylene (PE) with high carbon loading levels up to 40 to 60% by weight characterized by surface resistivity of about 10³ -10⁸ ohm/square. For applications requiring EMI shielding, weight loading levels of 30 to 50% PAN carbon fibers, 40% aluminum flakes, 15% nickel-coated carbon fibers or 5 to 10% stainless steel fibers have been used for the same class of polymers.

The method currently used to increase the electrical conductivity of polymers is to fill them with specific conductive additives, such as metallic powders, metallic fibers, carbon black, carbon fibers and recently by intrinsically conductive polymeric powders. The characteristic behavior of these materials is the existence of a strongly non-linear relationship between the electrical conductivity and the filler concentration. At low filler loading, the electrical conductivity of the polymeric compound is generally quite low; its magnitude is similar to that of the polymer matrix (10⁻¹⁶ to 10⁻¹¹ ohm⁻¹ cm⁻¹). As loading is increased, the conductivity increases sharply by several orders of magnitude over a narrow concentration range, then slowly increases towards the conductivity of the condensed filler powder of the order of 10⁻⁴ to 10⁻¹ ohm⁻¹ cm⁻¹. This behavior describes an insulator-conductor transition occurring at a critical volume fraction (percolation threshold). This threshold is due to the formation of a chain-like network of particles extending throughout the entire specimen volume and allowing electrical current to flow.

U.S. Pat. No. 4,169,816 describes an electrically conductive single thermoplastic material composition with a high carbon content, the composition including for each 100 parts of polypropylene-ethylene copolymer 15-30 parts of carbon black, 0.25 to 1 part of silica and 1-10 parts of a fiber reinforcing agent selected from carbon fibers or a mixture of carbon fibers and glass fibers.

U.S. Pat. No. 5,004,561 describes another single thermoplastic based electrically conductive composition with a high carbon content, the composition including for each 100 parts of thermoplastic resin selected from the group of polyolefin, polystyrene and acrylonitrite/styrene/butadiene (ABS) copolymer resin, polybutylene terephthalate (PBT) resin, polyphenylene ether and polyamide (PA) resin, 30-300 parts of electrically conductive glass fibers, 5-40 parts of carbon black and 5-40 parts of graphite.

Russian Patent No. SU 1,643,568 describes a high carbon based electrical conductive thermoplastic composition in which electrical conductivity is achieved from the dispersion of carbon in the matrix. The composition includes 20-35 weight percent polypropylene, 10-20 weight percent polyamide, 20-30 weight percent carbon black, 10-20 weight percent graphite and 15-20 percent glass fibers.

There are generally two methods for producing electrically conductive thermoplastic articles known in the art. In the slow production rate compression molding method less filler (e.g. carbon black) is required to achieve a desired conductivity, however the mechanical properties of the composition are usually deficient. In the fast production rate injection molding method better mechanical properties are achieved and articles having complex geometry can be produced but the amount of conductive filler required is high. One deficiency of compression molding of electrically conductive compounds is that the relatively slow processing is expensive.

A major disadvantage of prior art polymer based compounds for electrostatic dissipation and EMI shielding applications is the high percentage of conductive additives required to form the conductive polymer compounds resulting in high cost and deficient processability and mechanical properties and also high carbon contamination which is adverse in particular for clean room applications.

SUMMARY OF THE INVENTION

The present invention provides an improved thermoplastic electrically conductive composition.

According to an aspect of the present invention, the electrically conductive composition includes a first thermoplastic component forming a continuous matrix and a second thermoplastic component having a polarity larger than the polarity of the matrix. The composition also includes fibers being encapsulated in-situ by the second thermoplastic component and forming a network within the matrix and also a carbon black component which is preferentially attracted to the second component due to its higher polarity. The in-situ formation of an encapsulated network, including carbon black, in preferred locations of particles provides an electrically conductive composition.

According to a further aspect of the present invention, the ratio between the conductive carbon filler and the second component is sufficiently high so that a substantial part of the carbon filler is located at the interface between the second component and the matrix to provide the electrical conductivity. Nevertheless, the overall concentration of carbon is at least an order of magnitude smaller than in the prior art electrically conductive compositions, thus making the compositions of the present invention advantageous in many applications including clean room applications.

Another object of the present invention is to utilize a fast processing method for producing the thermoplastic electrically conductive compositions of the present invention. Injection molding is used for producing the electrically conductive thermoplastic compositions of the recent inventions while using very low carbon black concentrations and improving the mechanical properties of the composition.

The electrically conductive composition of the present invention includes a matrix including substantially a first thermoplastic component, a second thermoplastic component having a higher polarity than that of the first thermoplastic component, the second component encapsulating a plurality of fibers forming a network of encapsulated fibers within the matrix, and a carbon component preferentially attracted to the second component so as to make the network an electrically conductive network within the matrix.

In one embodiment of the present invention, the first thermoplastic component is a polyolefin compound with or without an added elastomer component. The polyolefin is selected from the group of polypropylene which may be a homopolymer or a copolymer and polyethylene. The second component is polyamide or EVOH. In a preferred embodiment, the composition includes less than 20 parts per hundred polyamide or EVOH.

In another embodiment, the first component is acrylonitrite/butadiene/styrene and the second component is polyamide, or EVOH.

In yet another embodiment, the first component is selected from polystyrene, high impact polystyrene and polyphenyleneoxide/polystyrene and the second component is polyamide or EVOH.

The fibers of the compositions of the present invention may be glass fibers. In a preferred embodiment, the composition includes less than 55 parts per hundred glass fibers.

The carbon component of the compositions of the invention of the present invention may be carbon black. Alternatively, or in combination, the carbon component may be carbon fibers. In a preferred embodiment, the composition includes less than 10 parts per hundred carbon black. In another preferred embodiment, the composition includes less than 30 parts per hundred carbon fibers.

In a preferred embodiment, the composition has one or more of a volume resistivity from about 0.1 to about 10⁹ ohm-cm, a flexural modulus of up to about 11,000 MPa, and a tensile strength of up to 60 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which:

FIG. 1 is a schematic illustration of the morphology of the electrically conductive thermoplastic compositions of the present invention;

FIGS. 2A-2C are nonlimiting exemplary SEM micrographs (at different magnifications) of freeze fractured surfaces of an injection molded composition of the present invention;

FIG. 3 is a schematic block diagram illustration of a method for producing the electrically conductive thermoplastic compositions of the present invention;

FIGS. 4A-4D are schematic block diagram illustrations of four alternative preferred methods for producing the electrically conductive thermoplastic compositions of the present invention wherein carbon black is the carbon compound; and

FIGS. 5A-5C are schematic block diagram illustrations of three alternative preferred methods for producing the electrically conductive thermoplastic compositions of the present invention wherein the carbon compound is both carbon black and carbon fibers.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Reference is now made to FIG. 1. FIG. 1 is a schematic illustration of the electrically conductive multi-component thermoplastic compositions of the present invention.

The electrically conductive multi-component thermoplastic composition, generally referenced 10, includes a matrix 12 formed substantially of a first thermoplastic compound, glass fibers 14 encapsulated with a second thermoplastic compound 16 having therein and thereon carbon black shown in the SEM micrographs of FIG. 2. As illustrated in FIG. 1, glass fibers 14 encapsulated with second thermoplastic composition 16 with carbon black thereon form a conductive network within polypropylene matrix 12.

In the preferred embodiments, the first thermoplastic component is a polyolefin based component, polystyrene (PS) based component or acrylonitrile/styrene/butadiene (ABS) terpolymer based compound, with or without an added elastomer component and the second thermoplastic component is polyamide (PA) or Polyethylene Vinyl Alcohol (EVOH) copolymer. Preferably, the elastomer component is a combination of elastomers. The addition of the elastomer component to the compound changes the composition's mechanical properties without significantly affecting its electrical conductivity.

Since the affinity of polyamide or EVOH 16 to glass fibers 14 is much stronger than to any of the first thermoplastic components, during the melt blending of composition 10, the second thermoplastic component) preferentially encapsulates, in situ, glass fibers 14 as described in detail with reference to FIG. 3 hereinbelow, thereby producing a network of encapuslated fibers within matrix 12. Moreover, carbon black particles are preferentially attracted to the second thermoplastic component phase and located at the second thermoplastic component (polyamide or EVOH) matrix interfaces as indicated by reference numeral 15, thereby making the network of encapsulated fibers electrically conductive using a much smaller content of carbon black than in the prior art and improving the mechanical properties of composition 10 as described hereinbelow.

Referring to the three Scanning Electron Microscope (SEM) micrographs of FIGS. 2A-2C which differ in their magnification, the network of encapsulated fibers and the preferential distribution of carbon black 18 in the second thermoplastic component phase 16 is clearly seen.

A particular feature of the present invention is that composition 10 includes much lower concentrations of carbon black than in the prior art for similar levels of electrical conductivities. This is since the electrically conductive carbon black particles form continuous pathways along the interfaces between the second thermoplastic component phase and the matrix (first level of percolation). Moreover, the embedded part of carbon black particles is located within the amorphous phase of the second thermoplastic 16 (second level of percolation), thereby the formation of the conductive pathways is facilitated. This is further facilitated by the network formed of second thermoplastic component encapsulated glass fibers (third level of percolation).

Reference is now made to FIGS. 3 through 5C which illustrate preferred methods for producing the electrically conductive thermoplastic compositions of the present invention. FIG. 3 illustrates the method in general and FIGS. 4A-4D and FIGS. 5A-5C illustrate, respectively, alternatives of the method for two nonlimiting exemplary compositions.

The method of FIG. 3 illustrated in a time sequence from left to right comprising the steps of compounding 32, the step of pelletizing 34 and the step of injection molding 36. In one preferred embodiment the step of compounding 32 includes the steps of dry blending of polypropylene with second thermoplastic component indicated by reference numeral 31 followed by melting and melt mixing with the glass fibers as indicated by 33 and subsequent melt mixing with carbon fibers, carbon black or both.

In the non limiting examples described below, the compounding step 32 is conducted on a twin-screw compounder (Berstorf, Germany), at processing temperatures in the range of 200-285° C. (corresponding to the melting point of the polymer components) and a screw speed of 55 rpm. The resulting compounds are pelletized (step 34) and then injection molded (step 36) at 200-285° C. on a Battenfeld injection molding machine equipped with a three cavity American Standard Testing Material (ASTM) mold (tensile bar, flexural bar and falling dart impact disc).

FIGS. 4A-4D illustrate four preferred alternatives of the method for producing composition 10 whereas the composition includes carbon black as the carbon compound. FIGS. 4A-4D are illustrated for the nonlimiting example of a composition 10 comprises 100 parts by weight polypropylene, 12 parts by weight polyamide, 30 parts by weight glass fibers, and 2 parts by weight carbon black.

In the embodiment illustrated in FIG. 4A, carbon black concentrate is added in the injection molding stage as indicated by reference numeral 37 rather than in the compounding step 32 resulting in a resistivity of 537 Ohm-cm and flexural modulus of 4819±161 MPa.

FIG. 4B illustrates the alternative wherein the carbon black concentrate is dry blended with the polypropylene and polyamide resulting in a resistivity of 432 Ohm-cm and flexural modulus of 4649±32 MPa. FIG. 4C illustrates the alternative wherein the carbon black concentrate is added during compounding resulting in a resistivity of 214 Ohm-cm and flexural modulus of 4491±51 MPa.

In the embodiment of FIG. 4D glass fibers are dry blended with the polypropylene and polyamide while carbon black concentrate is added during compounding resulting in a resistivity of 431 Ohm-cm and flexural modulus of 3790±63 MPa.

FIGS. 5A-5C illustrate three preferred alternatives of the method for producing composition 10 whereas the composition carbon fibers or carbon black and carbon fibers as the carbon compound. FIGS. 5A-5C are illustrated for the nonlimiting example of a composition 10 comprises 100 parts by weight polypropylene, 12 parts by polyamide , 30 parts by weight glass fibers, and 2 parts by weight carbon black and 20 parts by weight carbon fibers.

In the alternative of FIG. 5A both carbon black and carbon fibers are added during the injection molding step resulting in high conductivity (Resistivity of 0.465 Ohm-cm) and high flexural modulus (9770±428 MPa).

In the alternative of FIG. 5B carbon black is dry blended with the polyolefin and polyamide as indicated by step 39 instead of the dry blending indicated by step 31 of the other alternatives while carbon fibers are added (step 35) during compounding resulting in a resistivity of 2 Ohm-cm and flexural modulus of 9550±350 MPa.

In the alternative of FIG. 5C, carbon fibers are added in the compounding stage resulting in resistivity of 8 Ohm-cm illustrates and flexural modulus of 8931±267 MPa.

It will be appreciated that the specific method used to prepare the compounds forming composition 10 for injection molding can be selected from the non limiting embodiments described hereinabove and from many other variations thereof, thereby varying the specific electrical conductivity and mechanical properties (illustrated by the flexural modulus above) of composition 10. In all methods, the present invention provides an electrically conductive thermoplastic composition which provides high electrical conductivity and strong mechanical properties after injection molding.

The following examples illustrate without limitation certain aspects of the present invention.

In all examples, injection molded composite samples (12.6 cm×1.27 cm×0.32 cm) were characterized for electrical properties by measuring volume resistivity, according to ASTM D 257-93 and ASTM D 4496-87, using Keithley instruments. Silver paint was used to eliminate the contact resistance between samples and electrodes.

The corresponding ASTM test methods were used for the mechanical properties evaluation. Specifically, ASTM D 0638 was used for measuring tensile properties, ASTM D 790 was used for measuring flexure, ASTM D 256 was used to measure IZOD impact and ASTM D 570 was used to measure water absorption.

The glass fiber content and the carbon black content in each specimen was determined by using ASTM D 5630-94 and ASTM D 1603-94, respectively.

The composites morphology was studied using JEOL 5400 scanning electron microscope. Freeze fractured surfaces were studied.

In the experiments, commercial grades of PP (homopolymer and copolymer), ABS, PS, HIPS, NORYL (described in detail with reference to Tables 10-13), PE (high and low density grades), elastomer, PA (PA6, PA66, PA 11, PA12, PA 6/6.9, PA 6/12), EVOH, glass fibers (chopped strands, 3.2-6.3 mm length, 10-13 micrometer diameter), conductive carbon black and carbon fibers (chopped fibers, 6 mm length, 7-8 microns diameter) have been used in this study. Five types of carbon black were studied. Their properties are shown in Table 1 below.

                                      TABLE 1     __________________________________________________________________________                 Ketjen                      Ketjen EC                           Vulcan   Conductex 975                 EC 300J                      600JD                           XC-72                               Corax L6                                    (Columbian     Property    (AKZO)                      (AKZO)                           (Cabot)                               (Degussa)                                    Chemicals)     __________________________________________________________________________     Surface area BET, m.sup.2 /g                 1000      180 265  250     Particle size, nm                 30        29  18   22     Porevolume DBP, ml/100 g                 350  480  178 123  165     Surface area CTAB, m.sup.2 /g                 480       86  150  153     Iodine absorption, mg/g                 900  1000     293  260     Volatilities, %                 0.5  0.6  1.0 1.5  1.0     pH          8    8    7   7.5  7     __________________________________________________________________________

All blend ratios described relate to parts by weight, in each case based on 100 parts by weight of the first compound. An electrically conductive polyolefin composites comprise: 100 parts by weight polymer matrix, 4 to 20 parts by Polyamide, 10 to 55 parts by weight glass fibers, 0.5 to 10 parts by weight carbon black, 0 to 30 parts by weight carbon fibers.

The electrical and mechanical behavior of compositions 10 was studied for different polyamide mixtures ratios as a function of glass fiber concentration, polyamide/glass fiber ratio and carbon black concentration. The resistivity and the mechanical properties as a function of composition are summarized in Tables 2A and 2B. It will be appreciated that the samples produced were in accordance with the processing method of the present invention i.e., injection molding.

                                      TABLE 2A     __________________________________________________________________________     Compostion            1  2    3  4  5    6    7    8   9    10     __________________________________________________________________________     Polypropylene            100               100  100                       100                          100  100  100  100 100  100     (MFI 12)     Polyamide 6            5  5    5  5  5    8    8    10  12   10     Glass Fiber            12 12   20 20 20   20   30   30  30   30     (Vetro tex)     Carbon Black            1  3    0.5                       1  3    3    1    1.5 2    3     (EC 600)     Property     Flexural            3044               3849 3041                       3945                          4104 4366 5003 4655                                             4941 5153     Modulus,     MPa     Izod lmpact,            69 64   33 83 60   57   76   75  79   65     notched, J/m     Volume >10.sup.8               3.3 × 10.sup.5                    10.sup.8                       10.sup.4                          1.6 × 10.sup.5                               8.6 × 10.sup.3                                    6.6 × 10.sup.3                                         3 × 10.sup.3                                             1.4 × 10.sup.3                                                  6.5 × 10.sup.3     Resistivity     ohm-cm     __________________________________________________________________________

                                      TABLE 2B     __________________________________________________________________________     Composition 1    2    3    4    5    6    7     __________________________________________________________________________     Polypropylene (MFI 25)                 100  100  100  100  100  100  100     Polyamide 6 4    5.3  6.6  8    8    8    12     Glass fiber (Vetrotex)                 20   20   20   20   20   30   30     Carbon black (EC 600)                 0.67 088  1.1  1.5  2    1.3  2     Property     Specific Gravity, Kg/dm.sup.3                 1.02 1.01 1.02 1.02 1.02 1.06 1.06     Water Absorption, %                 0.04 0.06 0.06 0.04 0.05 0.08 0.08     Flexural Modulus, MPa                 3984 3958 3954 3941 3936 4866 5039     Tensile Strength, MPa                 54   58   52   52   58   63   63     Elongation at Break, %                 2.8  3.1  2.7  2.8  2.8  2.5  2.6     Izod Impact, notched, J/m                 68   85   66   63   71   85   76     Melt Flow Rate,                 5.2  4    6    2.8  3    3.3  3.2     230° C., 2.16 Kg     Heat Distortion Temp, ° C.                 161  163  161  162  163  164  163     Volume Resistivity,                 3.24 × 10.sup.5                      2.5 × 10.sup.3                           3.2 × 10.sup.4                                1.3 × 10.sup.3                                     7.9 × 10.sup.3                                          3.4 × 10.sup.3                                               1.2 × 10.sup.3     ohm-cm     __________________________________________________________________________

Tables 3A and 3B illustrate the dependence of resistivity and physical properties on glass fiber concentration in compositions 10 for different polypropylene/ polyamide mixtures. The increase of tensile strength, modulus and flexural modulus with increasing glass fiber content is clearly appreciated.

                  TABLE 3A     ______________________________________     Composition   1      2        3    4    5     ______________________________________     Polypropylene (MFI 12)                   100    100      100  100  100     Polyamide 66  5      8        12   16   16.5     Glass fiber (Vetrotex)                   2      20       30   40   50     Carbon black (EC600)                   1      1.5      2    2.8  2.8     Property     Tensile Strength, MPa                   39.4   44       56.2 58   47.4     Elongation at break, %                   3.2    2.6      2.3  2.1  2     Tensile Modulus, MPa                   1771   2030     2634 3133 2406     Flexural Modulus, MPa                   3054   3801     4684 5394 5576     Izod Impact, notched, J/m                   64     69       77   71   57     Water Absorption, %                   0.02   0.03     0.04 0.06 0.07     Specific Gravity, kg/dm.sup.3                   0.981  1.025    1.082                                        1.122                                             1.145     Volume Resistivity,                   >10.sup.8                          9.4 × 10.sup.4                                   850  440  2.3 × 10.sup.4     ohm-cm     ______________________________________

                                      TABLE 3B     __________________________________________________________________________     Composition 1   2    3    4  5    6  7  8  9     __________________________________________________________________________     Polypropylene (MF125)                 100 100  100  100                                  100  100                                          100                                             100                                                100     Polyamide 66                 4   5.3  6.6  8  8    6  8  10 12     Glass fiber (Vetrotex)                 20  20   20   20 20   30 30 30 30     Carbon black (EC 600)                 0.67                     0.88 1.1  1.5                                  2    1  1.3                                             1.6                                                2     Property     Specific Gravity, Kg/dm.sup.3                 1.02                     1.00 1.01 1.04                                  1.02 1.05                                          1.06                                             1.05                                                1.06     Water Absorption, %                 0.04                     0.04 0.07 0.04                                  0.05 0.05                                          0.07                                             0.1                                                0.05     Flexural Modulus, MPa                 3917                     3804 3858 373                                  3936 4815                                          4640                                             4732                                                4856                               1     Tensile Strength, MPa                 52  53   53   53 56   57 59 57 61     Elongation at Break, %                 2.7 3.1  3.0  3.0                                  3.0  2.6                                          2.4                                             2.9                                                2.0     Izod Impact, notched, J/m                 64  71   76   66 67   75 75 77 62     Melt Flow Rate,                 18  12   11   8.6                                  11   8  8.8                                             8  6     275° C., 2.16 Kg     Heat Distortion Temp, ° C.                 161 163  163  162                                  162  164                                          165                                             165                                                163     Volume Resistivity,                 2 × 10                     9.9 × 10.sup.3                          4 × 10.sup.3                               490                                  7.9 × 10.sup.3                                       650                                          814                                             241                                                175     ohm-cm      4     __________________________________________________________________________

Table 4 illustrates the dependency of electrical resistivity of PP/PA/GF/CB systems (100/12/30/4 phr ratio respectively) on the type of second thermoplastic component (polyamide) used. It is clearly seen that any type of polyamide can be used for composition 10 and that their electrical conductivity can be determined by selecting the degree of crystalinity of the polyamide. Lower values of resistivity were obtained for compositions 10 based on PA 66 and PA 6, PA11, PA 12 (semicrystalline polyamide) than for those based on PA 6/6-9 and PA 6-12 (amorphous polyamide). Given the same concentration of conductive additive, a higher level of conductivity was achieved in PA 66 (the most crystalline polyamide studied) based compound.

                  TABLE 4     ______________________________________                                            PA   PA     Property      PA 66   PA 6   PA12 PA11 6-6/9                                                 6-12     ______________________________________     Tensile Strength, MPa                   58.4    54.2   51.9 49.6 58.5 56.6     Elongationat break, %                   2.1     1.9    2    3    2.3  1.9     Tensile Modulus, MPa                   3256    3030   2541 1868 3074 3160     Flexural Modulus, MPa                   4850    4699   4592 3899 4650 4890     Izod Impact, notched, J/m                   72      70     68.8 58.9 61   62     Specific gravity, kg/dm.sup.3                   1.086   1.092  1.081                                       1.079                                            1.076                                                 1.069     Volume Resistivity,                   700     3.3 ×                                  2.3 ×                                       5.4 ×                                            3.3 ×                                                 1.3 ×     ohm-cm                10.sup.3                                  10.sup.3                                       10.sup.3                                            10.sup.5                                                 10.sup.5     ______________________________________

It will also be appreciated that any type of carbon black can be used with compositions 10. Resistivity and mechanical properties of composites with five CB grades, at two loading levels, for composites based on Polypropylene (100 parts), PA 66 (12 parts) and glass fibers (30 parts) are presented in Table 5. Inspection of this table reveals that Ketjenblacks EC 300 and EC 600, the most conductive carbon blacks used provide the highest electrical conductivity.

                                      TABLE 5     __________________________________________________________________________                  EC600                      EC 300                            XC 72 CORAX L6                                        CONDU CTEX     Property     2 phr                      2 phr                         4 phr                            2 phr                               4 phr                                  2 phr                                     4 phr                                        2 phr                                            4 phr     __________________________________________________________________________     Tensile Strength, MPa                  56.2                      52.1                         57.7                            45.9                               54 44.2                                     45.1                                        43.9                                            58.4     Elongation at break, %                  2.1 2.5                         2.3                            2.6                               2.3                                  2.6                                     2.2                                        3.1 2.1     Tensile Modulus, MPa                  3133                      2776                         2911                            2087                               2933                                  2291                                     2147                                        2193                                            3194     Flexural Modulus, MPa                  4684                      4898                         4803                            4968                               4747                                  5108                                     4335                                        4948                                            4638     Izod Impact, notched, J/m                  77  -- 72 70 68 74 66 79  72     Specific gravity, kg/dm.sup.3                  1.082                      1.072                         1.079                            1.079                               1.079                                  1.074                                     1.076                                        1.069                                            1.074     Volume Resistivity, ohm-cm                  850 1560                         789                            >10.sup.8                               4110                                  >10.sup.8                                     852                                        >10.sup.8                                            4 × 10.sup.4     __________________________________________________________________________

Yet another factor which determines the resistivity and mechanical properties of compositions 10 is the flowability (MFI) of the polymer matrix used. As clearly seen from Table 6 below, significant lower resistivities are obtained using for example polypropylene with higher MFI. Table 6 is provided below.

                                      TABLE 6     __________________________________________________________________________     Composition  1  2  3  4  5  6  7  8    9     __________________________________________________________________________     Polypropylene (MFI 3)                        100     Polypropylene (MFI 12)                  100      100   100   100     Polypropylene (MFI 25)                     100      100   100     100     Polyamide 66 5  5  12 12 12 16 16 16.5 15     Glass fiber (Vetrotex)                  12 12 30 30 30 40 40 50   50     Carbon black (EC 600)                  1  1  2  2  2  2.8                                    2.8                                       2.8  2.5     Property     Flexural Modulus, MPa                  3054                     3037                        4168                           4684                              4450                                 5812                                    5515                                       5576 6249     Volume Resistivity, ohm-cm                  10.sup.7                     2863                        1100                           850                              290                                 565                                    405                                       2.3 × 10.sup.4                                            543     __________________________________________________________________________

Table 7 hereinbelow provides examples of compositions 10 including carbon fibers as also illustrated with reference to FIGS. 5A-5C hereinabove. Volume resistivity of less than 1 ohm-cm were achieved in carbon fibers/carbon black/glass fiber/second thermoplastic component (polyamide)/polypropylene compounds. Table 7 below illustrates non limiting carbon fibers containing compositions and their properties.

                                      TABLE 7     __________________________________________________________________________     Composition  1  2  3  4  5   6  7  8  9  10     __________________________________________________________________________     Polypropylene (MFI 25)                  100                     100                        100                           100                              100 100                                     100                                        100                                           100                                              100     Polyamide 66 12 12 12 12 12  14 14 14 14 14     Glass Fiber (Vetrotex)                  30 30 30 30 30  40 40 40 40 40     Carbon Black (EC 600)                  2  2  2  2  2   2.8                                     2.8                                        2.8                                           2.8                                              2.8     Carbon Fiber 0  6  10 20 30  0  6  10 14 20     Property     Flexural Modulus, MPa                  4850                     6264                        7910                           9770                              10514                                  5964                                     6675                                        8129                                           9641                                              10495     Volume Resistivity, ohm-cm                  344                     7.2                        2  0.465                              0.168                                  425                                     12.2                                        1.2                                           0.96                                              0.32     __________________________________________________________________________

It will be appreciated that the compositions of the present invention, in particular the ones including carbon fibers and having a resistivity of less than 1 Ohm-cm provide improved Electro Magnetic Interference (EMI) shielding with a lower content of carbon black and carbon fibers. Therefore, they are superior to prior art electrically conductive plastics in a wide range of applications, such as clean room applications where the amount of carbon is important for the cleanliness of the process.

It will be appreciated that the present invention is not limited by what has been described hereinabove and that numerous modifications, all of which fall within the scope of the present invention, exist. For example, while the present invention has been described with reference to polypropylene, the present invention is equally applicable to other polyolefins such as polyethylene and to polyethylene-polypropylene mixtures. Polyethylene based compositions were prepared using LDPE and HDPE grades in a generally similar manner to that of polypropylene. As an example, a polyethylene based composition (100 parts on a similar weight basis) blended with 11 parts of polyamide (PA6), 20 parts of glass fibers and 4.4 parts of carbon black (EC-300) exhibit volume resistivity of 10⁵ -10⁶ ohm-cm.

Table 8 illustrates two further examples of polyethylene based compositions.

                  TABLE 8     ______________________________________     Composition         1          2     ______________________________________     PE 800 (MFI 20)     100     PE 600 (MFI 7)                 100     Polyamide 66        12         12     Glass fiber (Vetrotex)                         30         30     Carbon black (EC 600)                         2          2     Property     Specific Gravity, Kg/dm.sup.3                         1.07       1.07     Water Absorption, % 0.06       0.06     Flexural Modulus, MPa                         1663       1622     Tensile Modulus, MPa                         746        678     Tensile Strength, MPa                         19.3       17.1     Elongation at Break, %                         3.0        3.3     Izod Impact, notched,                         80         81     J/m     Melt Flow Rate, 275° C.,                         33.2       18.8     2.16 Kg     Heat Distortion Temp, ° C.                         96         96     Volume Resistivity, 1 × 10.sup.6                                    2 × 10.sup.6     ohm-cm     ______________________________________

In other alternative embodiments, the polyolefin component is replaced by another thermoplastic compound as the first component forming matrix 12. In four preferred embodiments, the first component forming matrix 12 was acrylonitrile/butadiene/styrene (ABS), polystyrene (PS), high impact polystyrene (HIPS) and polyphenyleneoxide/polystyrene (NORYL).

Table 9 illustrates two examples of polyethylene-polypropylene based compositions.

                  TABLE 9     ______________________________________     Composition         1          2     ______________________________________     PE 800 (MFI 20)     100     PE 600 (MFI 7)                 100     Polypropylene (MFI 25)                         24         24     Polyamide 66        8          8     Glass fiber (Vetrotex)                         20         20     Carbon black (EC 600)                         1.5        1.5     Property     Specific Gravity, Kg/dm.sup.3                         1.03       1.02     Water Absorption, % 0.05       0.05     Flexural Modulus, MPa                         1975       2117     Tensile Modulus, MPa                         1246       1176     Tensile Strength, MPa                         27         27.4     Elongation at Break, %                         2.3        2.5     Izod Impact, notched, J/m                         68         70     Melt Flow Rate, 275° C.,                         24.7       14.2     2.16 Kg     Heat Distortion Temp, ° C.                         101        102     Volume Resistivity, 3 × 10.sup.5                                    3 × 10.sup.5     ohm-cm     ______________________________________

Table 10 illustrates a nonlimiting example of compositions 10 formed with ABS, PS, HIPS and NORYL with different second thermoplastic component (different types of polyamide compositions, different glass fiber content and different low amounts (less than or equal to 3 phr) of carbon black.

                  TABLE 10     ______________________________________     Composition    1      2      3    4    5    6     ______________________________________     ABS            100    100    100     PS                                100     HIPS                                   100     NORYL                                       100     Polyamide 6           12               12     Polyamide 12   8             12   12     Polyamide 66                                12     Glass fiber (Vetrotex)                    20     30     30   30   30   30     Carbon black (EC 600)                    1.2    3      3    2    3    3     Property     Volume Resistivity, ohm-cm                    10.sup.8                           10.sup.4                                  10.sup.3                                       10.sup.5                                            10.sup.4                                                 10.sup.3     ______________________________________

Tables 11, 12 and 13 provide further examples of PS, HIPS and Noryl based compositions, respectively, and their mechanical properties.

Table 11 illustrates the dependence of resistivity and physical properties on glass fiber concentration in a PS/PA/GF/CB compound.

                  TABLE 11     ______________________________________     Composition  1        2        3      4     ______________________________________     PS (GP)      100      100      100    100     Polyamide 6  5        8        12     Polyamide 12                          12     Glass fiber (Vetrotex)                  12       20       30     30     Carbon black (EC 600)                  1.5      2        3      3     Property     Specific Gravity, Kg/dm.sup.3                  1.09     1.17     1.22   1.2     Water Absorption, %                  0.12     0.12     0.13   0.07     Flexural Modulus, MPa                  4850     5830     7100   7000     Tensile Modulus, MPa                  2740     3130     3650   3290     Tensile Strength, MPa                  56       70       66     59     Elongation at Break, %                  2        2        2      2     Izod Impact, notched,                  43       52       51     38     J/m     Melt Flow Rate,                  8.2      5.7      3.7    3.0     230 ° C., 2.16 Kg     Heat Distortion Temp,                  82       84       --     86     ° C.     Volume Resistivity,                  4.5 × 10.sup.5                           7.5 × 10.sup.4                                    3.8 × 10.sup.4                                           2.2 × 10.sup.4     ohm-cm     ______________________________________

Table 12 illustrates the dependence of resistivity and physical properties on glass fiber concentration in HIPS/PA/GF/CB compound.

                  TABLE 12     ______________________________________     Composition  1        2        3      4     ______________________________________     HIPS         100      100      100    100     Polyamide 6  5        8        12     Polyamide 12                          12     Glass fiber (Vetrotex)                  12       20       30     30     Carbon black (EC 600)                  1.5      2        3      3     Property     Specific Gravity, Kg/dm.sup.3                  1.12     1.16     1.21   1.19     Water Absorption, %                  0.13     0.14     0.14   0.07     Flexural Modulus, MPa                  3600     4560     6410   6050     Tensile Modulus, MPa                  1800     2050     2630   2050     Tensile Strength, MPa                  43       51       61     46     Elongation at Break, %                  2        2        2      2     Izod Impact, notched,                  46       46       47     34     J/m     Melt Flow Rate,                  2.3      1.7      1.9    1.7     230° C., 2.16 Kg     Heat Distortion Temp,                  --       97       92     93     ° C.     Volume Resistivity,                  9 × 10.sup.6                           7 × 10.sup.4                                    7 × 10.sup.3                                           1.5 × 10.sup.4     ohm-cm     ______________________________________

Table 13 illustrates the dependence of resistivity and physical properties on glass fiber concentration in Noryl/PA66/GF/CB compound.

                  TABLE 13     ______________________________________     Composition        1          2     ______________________________________     Noryl              100        100     Polyamide 66       12         16     Glass fiber (Vetrotex)                        30         40     Carbon black (EC 600)                        3          4     Property     Specific Gravity, Kg/dm.sup.3                        1.24       1.28     Water Absorption, %                        0.17       0.20     Flexural Modulus, MPa                        6501       7193     Tensile Modulus, MPa                        3382       4361     Tensile Strength, MPa                        64.1       79.2     Elongation at Break, %                        2.0        1.8     Izod Impact, notched, J/m                        38         42     Melt Flow Rate, 275° C.,                        1.0        0.5     2.16 Kg     Heat Distortion Temp, ° C.                        122        124     Volume Resistivity,                        5 × 10.sup.5                                   1.8 × 10.sup.4     ohm-cm     ______________________________________

Table 14 illustrates compositions 10 wherein the second thermoplastic component is EVOH and not polyamide. The table illustrates the dependence of resistivity on glass fiber concentration in PP/EVOH/GF/CB compound.

                  TABLE 14     ______________________________________     Composition    1         2         3     ______________________________________     Polypropylene (MFI 25)                    100       100       100     EVOH           8         10        12     Glass fiber (Vetrotex)                    20        30        30     Carbon black (EC 600)                    2         2.5       3     Property     Specific Gravity, Kg/dm.sup.3                    1.03      1.07      1.07     Water Absorption, %                    0.03      0.03      0.05     Flexural Modulus, MPa                    4530      5697      5529     Tensile Modulus, MPa                    2475      2572      2670     Tensile Strength, MPa                    59.7      62.1      61.8     Elongation at Break, %                    2.7       2.5       2.4     Izod Impact, notched, J/m                    69        66        58     Melt Flow Rate, 230° C.,                    5.0       4.5       3.2     2.16 Kg     Heat Distortion Temp, ° C.                    160       160       160     Volume Resistivity,                    1.7 × 10.sup.5                              1.6 × 10.sup.5                                        1.2 × 10.sup.3     ohm-cm     ______________________________________

Table 15 illustrates the dependence and resistivity and physical properties on glass fiber concentration in PP/PA/GF/CB (PP: Elastomer 60:40) compound containing elastomer. It is clearly shown that the compounds containing elastomer offer higher impact than the compounds summarized in Table 3B.

                  TABLE 15     ______________________________________     Composition      1       2       3     4     ______________________________________     Polypropylene    100     100     100   100     Polyamide 66     5       8       8     10     Glass fiber (Vetrotex)                      12      20      25    30     Carbon black (EC600)                      1       1.5     1.5   2     Property     Flexural Modulus, MPa                      1500    1900    2000  2300     Izod Impact, notched, J/m                      134     120     107   100     Surface Resistivity, ohm/sq.                      10.sup.7                              10.sup.6                                      10.sup.5                                            10.sup.5     ______________________________________

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention is defined only by the claims that follow: 

We claim:
 1. An electrically conductive composition comprising:a matrix comprising substantially a first thermoplastic component; a second thermoplastic component having a higher polarity than that of said first thermoplastic component, said second component encapsulating a plurality of fibers forming a network of encapsulated fibers within said matrix; and a carbon component preferentially attracted to said second component so as to make said network an electrically conductive network within said matrix, said carbon component located at the interface between said second thermoplastic component and said matrix wherein said carbon component is carbon black and is present in an amount less than 10 parts per hundred.
 2. A composition according to claim 1 wherein said first thermoplastic component is a polyolefin compound with or without an added elastomer component.
 3. A composition according to claim 2 wherein said polyolefin is selected from the group consisting of polypropylene and polyethylene.
 4. A composition according to claim 3 wherein said polypropylene is a homopolymer or a copolymer.
 5. A composition according to claim 1 wherein said second component is polyamide or EVOH.
 6. A composition according to claim 5 wherein said composition comprises less than 20 parts per hundred polyamide or EVOH.
 7. A composition according to claim 1 wherein said fibers are glass fibers.
 8. A composition according to claim 7 wherein said composition comprises less than 55 parts per hundred glass fibers.
 9. A composition according to claim 1 wherein said carbon component further carbon fibers.
 10. A composition according to claim 1 wherein said carbon component is a mixture of carbon black and carbon fibers.
 11. A composition according to claim 9 comprising less than 30 parts per hundred carbon fibers.
 12. A composition according to claim 1 having a volume resistivity from about 0.1 to about 10⁹ ohm-cm.
 13. A composition according to claim 1 having a flexural modulus of up to about 11,000 MPa.
 14. A composition according to claim 1 having a tensile strength of up to 60 MPa.
 15. A composition according to claim 1 wherein said first component is selected from acrylonitrite/butadiene/styrene, polystyrene, high impact polystyrene and polyphethyleneoxide/polystyrene.
 16. A composition according to claim 15 wherein said second component is polyamide or EVOH.
 17. A composition according to claim 2 wherein said elastomer component comprises a combination of elastomers.
 18. A composition according to claim 17 wherein said combination of elastomers affect the mechanical properties of said composition without significantly affecting its electrical conductivity.
 19. A method for producing an electrically conductive composition comprising the step of compounding a matrix comprising substantially a first thermoplastic component with a second thermoplastic component having a higher polarity than that of said first thermoplastic component, said second component encapsulating a plurality of fibers forming a network of encapsulate fibers within said matrix; anda carbon component preferably attracted to said second component so as to make said network an electrically conductive network within said matrix, said carbon component located at the interface between said second thermoplastic component and said matrix wherein said carbon component is carbon black and is present in an amount less than 10 parts per hundred.
 20. A method according to claim 19 further comprising the step of forming said first thermoplastic component from a polyolefin compound with or without an added elastomer component.
 21. A method according to claim 20 wherein said polyolefin is selected from the group consisting of polypropylene and polyethylene.
 22. A composition according to claim 21 wherein said polypropylene is a homopolymer or a copolymer.
 23. A composition according to claim 19 further comprising the step of forming said second thermoplastic component from polyamide or EVOH.
 24. A method according to claim 23 wherein said composition comprises less than 20 parts per hundred polyamide.
 25. A method according to claim 19 wherein said fibers are glass fibers.
 26. A method according to claim 25 wherein said composition comprises less than 55 parts per hundred glass fibers.
 27. A method according to claim 19 wherein said carbon component further carbon fibers.
 28. A method according to claim 19 wherein said carbon component is a mixture of carbon black and carbon fibers.
 29. A method according to claim 27 wherein said composition comprising less than 30 parts per hundred carbon fibers.
 30. A method according to claim 19 wherein said composition has a volume resistivity from about 0.1 to about 10⁹ ohm-cm.
 31. A method according to claim 19 wherein said composition has a flexural modulus of up to about 11,000 MPa.
 32. A method according to claim 19 wherein said composition has a tensile strength of up to 60 MPa.
 33. A method according to claim 19 comprising the step of forming said first component from acrylonitrite/butadiene/styrene, polystyrene, high impact polystyrene and polyphenyleneoxide/polystyrene.
 34. A method according to claim 33 wherein said second component is polyamide or EVOH.
 35. A method according to claim 19 wherein said elastomer component comprises a combination of elastomers.
 36. A method according to claim 35 wherein said combination of elastomers affect the mechanical properties of said composition without significantly affecting its electrical conductivity. 